This might just be the best advice you could get for now. Also, remember you're mapping on rather small islands, I don't think you'll need a denser motorway network that in the Netherlands (especially not in that part of the world I think). Take a look at a few real world overpopulated islands that might fit the general atmosphere of Tarephia:

And don't forget to respect the culture of the area, I think it's not ok to have an American-styled Island next to a culturally very harmonically created continent. If I was you, I'd try to look at these incredible OGF examples to find the fitting culture for your islands:

If I might start with the terrain: The Murakami River is quite curious in that it travels south so far from a mountain peak instead of flowing westward through Antochic. I think there's a bit of a stretch there. If it were the case that it flows south, there would definitely need to be some type of rifting, cliffs, or canyons that prevent it from flowing west. This would be an incredibly interesting geological feature that would definitely show up on the map: roads would not cross it, trains would be unable to cross it, it might be geologically active so no real development around it, etc. The Coast Motorway, in particular, is quite a difficult sell given that and other topographical features you have.

I would suggest not continuing your grid system up the mountain or deep into the forests. If you look at an area in real life near New York City, almost no road systems, much less grid systems, pass through it. I would say that if you want to put your grids there. you should think from a planner's perspective and just cut the forest down. If this is not economically feasible, just don't go through it. I do have some roads near the borders of my country that go through forests or large fields, but these are usually minimal, and any structures built there are in cleared land.

Also, as an island nation, you should think about what kind of people would come out from that kind of land and what kind of economic output they could produce. That huge city you made was built using money. You need to create a way for your economy to support and build up such a city. As an island nation, you don't have many resources, but your country has many other ways to get money. You could go like Singapore and be based on trade and shipbuilding, or like the Bahamas and go out all on the environment and tourism. In any case, think about how that city was built and what funded it.

Main points:

Don't put grids up mountains or through forests. Any roads or rails going through there must be minimal. If you really want a grid there, remove the forest.

Think of a way that your country could have funded the construction of such a large city. Build some shipyards, resorts, farms, etc. A city can't be built without money.

Respect your nature. Island nations, especially tropical island nations, are usually very underdeveloped and there are lots of natural resources and rainforests in them.

Think like an islander. Try building only around the edges of the island rather than the centre to be realistic.